ndisc: fix ordering of routes

When no existing route has lower priority than the new one, insert
at the end of the array instead of at the beginning. This is the
same fix that was applied to gateways.

Fixes: b324b970bc ('rdisc: fix double-addition of gateways & routes if priority increases')

src/core/ndisc/nm-ndisc.c

@@ -768,7 +768,7 @@ nm_ndisc_add_route(NMNDisc *ndisc, const NMNDiscRoute *new_item, gint64 now_msec
         return FALSE;
     }
 
-    g_array_insert_val(rdata->routes, insert_idx == G_MAXUINT ? 0u : insert_idx, *new_item);
+    g_array_insert_val(rdata->routes, insert_idx == G_MAXUINT ? rdata->routes->len : insert_idx, *new_item);
     return TRUE;
 }